The present invention relates to methods for gravel packing of wellbores penetrating subterranean formations and particularly for gravel packing highly deviated wellbores.
Petroleum recovery operations, such as well stimulation and gravel packing, often require the use of fluid compositions capable of suspending particles. In gravel packing operations, a pack of gravel is placed on the exterior of a perforated or slotted liner or screen which is positioned across an unconsolidated formation. The resulting structure presents a barrier to migrating sand from the formation while still permitting fluid flow. The gravel is carried to the formation in the form of a slurry by mixing gravel with a viscosified fluid. Once the gravel is placed in the wellbore, the viscosified carrier fluid is degraded and returned to the surface.
Prior to effecting the gravel pack, drilling mud and other contaminants may be washed from the wellbore and the formation treated. Commonly employed treatments include acidizing to dissolve formation clays and injecting stabilizing agents to prevent migration of formation components.
While numerous methods are available for effecting gravel packs in substantially vertical wellbores, such methods often are unsatisfactory in effecting gravel packing of highly deviated wellbores. Conventional gravel packing fluids utilizing uncrosslinked hydroxyethyl cellulose, hydroxypropyl guar, xanthan gum and the like as the viscosifier allows or permit the gravel to fall or settle to the low side of the tubing in long highly deviated wells. Such settling can result in a premature "sand-out" caused by a bridging of the settled particles across the tubing. Mechanical devices such as a baffled washpipe and oversized washpipes have been used to attempt to prevent or minimize the premature settling of the gravel. Such devices have experienced only limited success in solving the problem.
An ideal fluid for gravel packing operations would be one that shows little or no settling of gravel so that a high concentration of gravel can be transported through the tubing at any angle. The fluid also should exhibit adequate fluid loss into the formation matrix to insure compact packing of the gravel against the formation face. The fluid also should "break" to a reduced viscosity fluid similar to the viscosity of water over a predesigned time interval and deposit no residual solids so as to avoid or minimize any formation damage.
Gravel packing of wells which are highly deviated necessitate the use of a viscosified fluid which is capable of providing an infinite gravel fall rate, that is a substantially zero rate of gravel settling in the fluid during placement in the wellbore. A gravel packing fluid having a substantially zero gravel fall rate would assure that the gravel being carried to the production zone in a highly deviated wellbore would not prematurely settle-out.
One method employed to substantially achieve a zero gravel fall rate with a polymeric gravel packing fluid is to crosslink the polymer. An aqueous solution of a crosslinked polymer provides a gel structure which suspends the gravel particles causing substantially a zero fall rate. Polymers which typically have been utilized are guar, hydroxypropyl guar and carboxymethylhydroxypropyl guar and derivatized cellulose such as carboxymethylhydroxyethyl cellulose because of their ease of crosslinking with metal ions such as titanium (IV) and aluminum (III). The main objections to using such fluids for gravel packing operations are the limited amount of fluid loss to the formation which may cause voids within the pack and the excessive amount of formation damage which can result from use of the fluids due to water insoluble particles, particularly such as those found in guar based derivatives. The water insoluble particles are believed to remain in the formation and may cause formation plugging or impair the permeability of sand or gravel packs.
Cellulose derivatives are a preferred viscosifying polymer for certain petroleum recovery operations because they degrade, i.e., lose viscosity while generating only limited amounts of water insoluble particles or residue. Cellulose derivatives have had limited use in many petroleum applications because most derivatives are salt sensitive or not crosslinkable. Common non-ionic derivatives of cellulose are generally not crosslinkable because the polymer lacks a site for attachment of a multivalent metal cation. Examples of this type include hydroxyalkyl cellulose ethers, methyl cellulose, ethyl cellulose and hydroxyalkylmethyl cellulose. A crosslinkable non-ionic cellulose derivative has been prepared and described in U.S. Pat. Nos. 4,523,010 and 4,552,215 which are herein incorporated by reference. In these disclosures, dihydroxypropylhydroxyalkyl cellulose is prepared by a condensation reaction of glycidol with hydroxyethyl cellulose (HEC) under alkaline conditions. The glycidol addition along the HEC polymer chain provides a site of attachment for multivalent metal cations.
Anionic cellulose derivatives are normally substituted with carboxyl groups along the polymer chain. The carboxyl groups complex with polyvalent metal cations, such as aluminum. Gels formed with this chemistry tend to have limited structural stability at formation temperatures of about 200.degree. F. In addition, carboxylate substituents render the polymer salt sensitive, i.e., the viscosity of the polymer in a salt solution is less than the viscosity in water. Salt sensitivity is not a desirable property because the aqueous liquids used in recovery operations most generally contain chloride salts to inhibit the swelling of formation clays.
The present invention provides new and useful methods of effecting gravel packing through use of new methods of crosslinking in selected graft copolymers of cellulose derivatives, which are generally non-ionic in character. Methods of grafting monomers on polyhydroxy containing compounds are well known in the art. The process is described in U.S. Pat. No. 2,922,768, herein incorporated by reference. U.S. Pat. Nos. 4,982,783; 5,067,565; and 5,122,549, the entire disclosures of which are incorporated herein by reference, describe processes by which crosslinkable cellulose derivatives are prepared by grafting vinyl or allyl monomers having a crosslinkable substituent onto the cellulose derivative. The resulting copolymer is non-ionic and crosslinks readily with polyvalent metal cations to form stable viscoelastic gels.